Process for the protection of magnesium-base metals



Patented Jan. 11, 1944 PROCESS FOR THE PROTEGTION OF MAGNESIUM-BASE METALS Jean Frasch, Clichy, France; vested in the Alien Property Custodian No Drawing. Application February 21, 1939, Se-

rial No. 257,762.

In France February 25, 1938 9 Claims. l'Cl. 204-56) The object of the present invention is a process for the protection of magnesium and magnesium-base alloys by means of a superficial coating.

The anodic or electrolytic treatment-of magnesium and its alloys has heretofore generally been effected in alkaline solutions with a view to depositing coatings of oxide, silicates or aluminates, the said solutions sometimes containing in addition other alkaline salts such as borates, chromates, phosphates and molybdates. But coatings so obtained have never given entire satisfaction.

Up till now acid solutions have not successfully been employed for the anodic, or electrolytic treatment of magnesium and its alloys. Such solutions have only been used in the form of bichromates to which concentrated nitric acid has been added in order to deposit a golden surface film, or of boiling chromic acid for the purpose of cleaning the surface of the metal and dissolving the oxide deposit which is generally present.

Recently attempts have been made to protect magnesium and its alloys anodically or electrolytically by means of solutions of phosphates, sulfates or other alkaline salts-to which CrO, K Cr O' or KCrO have been added. Such solutions deposit on the surface of the metaLin ada to such a solution small quantities of sulfuric acid or chromium sulfate, preferably in the ratio of 0.5 to 3%, calculated as S0 to CrO present.

The pH of the bath should be maintained at less than 2. This may be done by the appropriate addition from time to time of chromic acid and if necessary of sulfuricacid.

I have found that when magnesium or its alloys are treated with alternating current in such a bath a dark brown or black coating is deposited on the surface of the metal.

tween CrO and CrO and contains practically no metal other than chromium.

This film consists' of an oxyde of chromium, intermediate be- I have further made the surprising discovery that the above mentioned protective film is porous and far from being of an insulating nature, easily permits the passage of alternating electric current. The voltage across the bath only falls slightly during the electrolytic .treatment and remains almost constant for its duration.

Thus, for example, if one applies a potential of 2 to 10 volts between the electrodes, regulating the current at from 2 to 15 amps. per dmfi, I have observed, in contradistinction to all other heretofore known electrolytic processes for protecting magnesium and its alloys, that the current only falls slightly during the treatment, the dark film formed on the metal offering but little resistance to the passage of the current. For example an initial voltage of 5 voltswill. fall to about 4.5 volts at the end of the treatment.

I give below the composition of two baths particularly well suited for putting my present invention into effect, but these baths are cited merely by way of example, and all baths which conform to the characteristics previously cited are capable of being utilised.

Example 1 Composition of bath:

' Cr0 grs. per litre 300 HSO do 5 Voltage volts 4 Current -amp./dm.=.. 8 Time minutes 10 Temperature Room temperature Example 2 Composition of bath:

Cr0 grs. per litre-.. 50 Voltage volts..' 8 Current amp./dm. 4 Time minutes 15 Temperature Room temperature After the electrolytic treatment the objects are withdrawn from the bath, copiously rinsed and dried, preferably hot. Subsequently the objects can be subjected to a finishing treatment, for example by means of paraffin, paint, etc., which increases the resistance of the protective coating to corrosion.

In particular, since the protective film is porous, one obtains excellent results by means of immersion of the pieces in hot paraflin, for example at about C.

The previously described bath containing chromic and sulphuric acids has moreover the advantage of being one well suited to the preliminary pickling of the metal objects to be electrolytically treated, with the result that no prior pickling is necessary.

I have also discovered that magnesium or magnesium-base alloys assembled with an other metal such as iron or aluminium, can 'be satisfactorily treated in the above mentioned bath.

My invention covers, as new industrial products, not only baths as specified for the electrolytic treatment of magnesium and magneslum-base alloys with a view to producing protective coatings resistant to corrosion, but also all objects in magnesium or its alloys which are covered with the afore described protective coating consisting substantially of an oxyde of chromium, and this protective coating itself.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is performed, I declare that what I claim is:

.1. A-process for producing on magnesium-base metals coatings resistantto corrosion containing an oxide of chromium substantially uncontaminated by any other metal, in which the metal to be protected is electrolytically treated at'normal temperature with a alternating current of a voltage between 2 and 10' in a bath containing chromic acid, the pH oi. the bath 9 being below 2.

2. A process according to claim 1 in which the pH of the bath is maintained below 2 by further addition of acid.

. 3. A process according to claim 1 in which the pH of the bath is maintained below 2 by further addition of chromic acid.

4. A process according to claim 1 in which the bath contains no salt or oxyde capable of forming on magnesium an oxyde or salt of a metal other than chromium.

5; A process according to claim 1 in which the bath also contains 80* ions.

6. A process according to claim 1 in which the bath contains sulfuric acid, the ratio of suliuric to chromic-acidvaryin'g from 0.5 to 3%.

'i. A process according to claim 1 in which the objects to be protected are electrolytically treated without prior pickling.

8. Objects in magnesium and magnesiumbase alloys whenever covered with a blackish brown protective coating which contains an oxyde of chromium substantially uncontaminated by any other metal.

9. Objects in magnesium and magnesium-base alloys whenever protected with a blackish brown coating containing an oxide of chromium substantially uncontaminated by magnesium and pervious to alternating electric current.

JEAN FRASCH. 

